Rare earth magnet is based on intermetallic compound R2T14B, thereinto, R is rare earth element, T is iron or transition metal element replacing iron or part of iron, B is boron, Rare earth magnet is called the king of the magnet with excellent magnetic properties, the max magnetic energy product (BH)max is ten times higher than that of the ferrite magnet (Ferrite), besides, the rare earth magnet has good machining property, the operation temperature can reach 200° C., it has a hard quality, a stable performance, a high cost performance and a wide applicability.
There are two types of rare earth magnets depending on the manufacturing method: one is sintered magnet and the other one is bonded magnet. The sintered magnet has wider applications. In the conventional technique, the process of sintering the rare earth magnet is normally performed as follows: raw material preparing→melting→casting→hydrogen decrepitation (HD)→jet milling (JM)→compacting under a magnetic field→sintering→heat treatment→magnetic property evaluation→oxygen content evaluation of the sintered magnet.
Crushing method of rare earth magnet is usually applied with a two-stage crushing method: hydrogen decrepitation (HD) and jet milling (JM). Hydrogen decrepitation (HD) is a method that for the rare earth magnet alloy (for example NdFeB magnet alloy) to absorb hydrogen, with the absorption of hydrogen, the hydrogen absorption part of the alloy may expand so that the inner of the alloy breaks or cracks, that is a relatively simple grinding method. Jet milling (JM) is a method for ultrasonically accelerating the powder in almost no oxygen atmosphere, the powders impact mutually, then the impacted powder is classified as desirable powder and R rich ultra fine powder (below 1 μm). It is a common belief that jet milling is a necessary process, the reason is that, the powder with certain centralized particle size distribution may improve the compacting property, orientation, coercivity and other magnet properties.
Compared to other powder particles with less content of rare earth element R (with larger particle size), R rich ultra fine powder is oxygenated more easily, if sintering the green compacts without removing the R rich ultra fine powder, the rare earth element may be significantly oxygenated in the sintering process, resulting in low production of crystallization phase with main phase R2T14B as rare earth element R is used to bind with oxygen. However, the process of removing ultra fine powder needs powder classifying device, special filter to recycle the inert gases and other complicated devices. The classifying process in jet milling methods needs a screen shape rotating blade with a high rotating speed, however, to ensure a stable rotating speed in 3000 rpm˜5000 rpm, it may cause the consumption of the rotating blade, bearing and other precise components. Besides, the departed ultra fine powder of the rare earth magnet alloy may be easily reacted with oxygen and burn fiercely that brings danger to the operators when cleaning the jet milling device.
With the continuous development of low oxygenation technique in the rare earth magnet manufacturing and the continuous improvement of the air-tightness technique from the compacting to the sintering processes, oxygenation may rarely happens during from the compacting to the sintering processes. Therefore, oxygenation may mainly happen during the jet milling process that needs large amount of jet steam, for example, when the oxygen content in the jet milling is about 10000 ppm, the oxygen content of the obtained sintered magnet is about 2900 ppm˜5300 ppm; however, for obtaining the sintered magnet with a lower oxygen content by decreasing the oxygen content of the jet steam, there may need to increase the investment cost and the manufacturing cost.
In addition, as rare earth resource is continuously reduced with continuous mining, rare earth is more and more precious, so that it has to efficiently use the rare earth. A loss of about 0.5˜3% of the powder in the jet milling process may gradually become a problem.